1. Field of the Invention
This invention relates to an optical recording medium adapted to record information in a recording layer by irradiating a light beam thereto to induce a crystallographic change. It also relates to a method for determining the activation energy of thin films such as phase change films and magneto-optical recording films of optical recording media.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting. One typical rewritable optical recording medium is of the phase change type wherein laser light is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by a single light beam and operated by a drive unit with a relatively simple optical system as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Ge--Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as defined in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe.sub.2 phase.
Japanese Patent Application Nos. 108996/1992, 179267/1992, 253832/1992, 17968/1993, 341818/1993, and 87854/1994 by the inventors disclose phase change optical recording media having recording layers based on Ag, Sb, Te, and In to which V, Ti or the like is added for improving reliability and other properties. However, these media which have been recorded at a high linear velocity are less reliable when stored at elevated temperature. For example, those media having vanadium added to the recording layer for reliability improvement which are recorded at a linear velocity of 4 m/s or higher have the problem that amorphous record marks crystallize in about 200 hours during storage in an environment at 80.degree. C. Japanese Patent Application Nos. 179267/1992 and 17968/1993 have a corresponding U.S. Pat. No. 5,418,030. Japanese Patent Application No. 253832/1992 has a corresponding U.S. Pat. No. 5,389,417. Japanese Patent Application No. 341818/1993 has a corresponding U.S. Pat. No. 5,470,628. Japanese Patent Application No. 87854/1994 has a corresponding U.S. patent application Ser. No. 08/400,765.